Sexing bettas comes up a lot - after all, how many of us have bought what we think is a girl, and later turns out to be a boy? So I thought I'd make a thread on ways to sex your betta.
1) Fin size
This is the most obvious factor, and one that most people rely on. Of course, it can be misleading. Typically, males will have large, beautiful finnage, whilst females will have much shorter fins. This can, of course, be misleading, with some females having unusually large fins, and certain males, such as plakats, having much smaller fins.
The next thing to consider is the size of the ventrals (the long dangly fins just behind the gills). The ventrals on a male are typically much thicker and longer. This is most obvious when they flare. Again, however, it is not decisive.
When a male flares, his beard will extent under his chin and well as in a broad mane around the sides of his head. Females only have little manes, with little to no beard under the chin.
The ovipostor, or egg-spot, is a little white dot seen between the ventral fins. However, it's not a great in determining gender. Not all females display them, and juvenile males sometimes have them.
5) Shape of the anal fin (that's the one under the body)
So, you've found a fish who could be a male plakat, or could be a female, you aren't sure. Well, females generally have quite boxy anal fins that form a relatively straight line under the body. A plakat male will have a longer anal fin that slopes down to a point near his tail.
Don't take aggressiveness as a sign of gender. In my experience, males are more aggressive, but females round my way are bred pretty docile. Some females can be just as feisty as males!
Bubblenests are generally a male trait, but some males won't build them, and some females will! They are a guide, but not determinative.
I hope this helps. :) I'm sorry, I have no idea how to embed images in my text, but if someone wants to take this text and make a post with pictures illustrating what I mean, you are more than welcome to do so! (Just give me credit ;)).